Tester found A very critical bug in DB testing. DB testing was out of scope. He started by himself and found a critical bug. How should I write an appreciation mail?
Assuming that you're mentoring the Tester whom you want to appreciate for his work the email would be like as below:-
Thanks for improving the value of our product by finding this XX critical Bug. Thank you for taking initiative and going beyond the scope in your testing. We encourage your Out of the box nature and heartfully appreciate your efforts and wish to see more from good work from you.
As a token of appreciation for your great work we have decided to change your Job title and increase your salary by xxxx.